This invention relates to pneumatic tires, and more particularly to an improvement of a bead portion in pneumatic radial tires for heavy vehicles.
In general, a tire of this type comprises as shown in FIG. 1 a carcass 1 of one ply alone or possibly two or more plies arranged in a substantially radial direction of the tire and is provided with a rubber stock S disposed above a bead core 2 between the carcass 1 and the turnup portion 1' thereof. The stock S is a substantially triangular section, and a reinforcing layer 31 is arranged outside the turnup portion 1' of the carcass 1 and composed of metal cords each beingcrossed at a relatively large angle (about 60.degree.) with respect to the radial direction of the tire. Two reinforcing layers 32, 33 are arranged outside the reinforcing layer 31, each composed of organic fiber cords crossed at a relatively large angle (about 60.degree.) with respect to the radial direction of the tire because the rigidity of the bead portion is fairly small when compared with the case of bias tires. However, the tire of this structure satisfies the requirement of rim chafing resistance to a certain extent, but does not always satisfy the requirement of durability of the bead portion because of peeling off of each reinforcing layer 31, 32, 33 at upward end thereof (hereinafter referred to as separation) is apt to be caused due to the stress concentration during the rotation of the tire under a load.
It is, therefore, an object of the invention to propose developmental results on the reinforcing structure of the bead portion, in which the separation of the bead portion is effectively prevented without lowering the rimchafing resistance and hence the durability of the bead portion is considerably improved by investigating the appropriate arrangement and construction of the above mentioned reinforcing layers in the bead portion.
The inventors have made detailed studies and found out that the object of the invention can advantageously be achieved by the following construction.
That is, according to the invention there is provided a pneumatic radial tire for heavy vehicles comprising a carcass composed of at least one ply of cords, each being arranged in a radial plane inclusive of the rotary axis of the tire or inclined at an extremely small angle with respect to the radial plane. The carcass ply is wound around a bead core from the inside toward the outside thereof to form a turnup portion to sandwich a rubber stock disosed on the bead core, and a plurality of reinforcing layers for bead portion each containing cords crossed with the cords of the carcass ply. The improvement comprises that the reinforcing layers have heights which are different in the upward end as measured from a reference line parallel to the rotary axis of the tire passing through the center of the bead core toward the radial direction of the tire and have cords with substantially the same modulus of elasticity that are crossed with each other such that the cords of the reinforcing layers are inclined at a larger angle with respect to the radial plane of the tire as the height of the radially upward end becomes lower.